1. Field of the Invention
The present invention relates to a method of polarization-treating a piezoelectric body for use in a ceramic filter, a ceramic oscillator, and other suitable devices.
2. Description of the Related Art
For polarization-treatment of piezoelectric ceramic substrates (block, unit, or other forms) of PZT and PT types, conventionally, after a piezoelectric ceramic substrate is fired, electrodes are provided on both of the opposite surfaces of the piezoelectric ceramic substrate. A plurality of piezoelectric ceramic substrates are simultaneously dipped into a polarization liquid at a temperature of 60xc2x0 C. to 100xc2x0 C., and a voltage of 2 kV/mm to 8 kV/mm is applied for about 10 to 30 minutes to obtain a desired polarization degree.
Polarization treatment of a piezoelectric body may be performed by in-liquid polarization which is carried out in a polarization liquid having insulation properties as described above, in-air polarization which is conducted in the atmosphere or in an gas atmosphere. In the in-air polarization, a desired electric field intensity can not be attained, since discharge occurs at a voltage of about 1 kV/mm or higher. For this reason, in general, the in-liquid polarization is carried out in order to obtain a high polarization degree.
However, in the case of the in-liquid polarization, the polarization degree of a piezoelectric body can not be measured during polarization. This is because the vibration of the piezoelectric body placed in the liquid is damped, due to the liquid, and the frequency characteristic can not be measured. For this reason, conventionally, the in-liquid polarization is a constant-time polarization in which the polarization is carried out for a predetermined time period. As a result, there arises the problem that the polarization degree can not be exactly controlled, causing the firing and composition of the piezoelectric body to be dispersed, which results in dispersion of the polarization degree.
In Japanese Patent No. 2656041, a polarization method includes measuring a piezoelectric constant (for example, electromechanical coupling coefficient K) during polarization, and the application of a voltage is stopped when the constant reaches the predetermined level which is decided by a correlation between the value K obtained immediately after the polarization is stopped and the stable value K obtained after a lapse of time. By this, dispersion of the polarization degree, caused by dispersion of materials and firing conditions, can be reduced. Accordingly, a piezoelectric body having constant qualities can be produced.
However, in the above-described method, it is necessary to determine the piezoelectric constant value based on the frequency during the polarization, which requires a long measurement time. Accordingly, when many piezoelectric bodies are simultaneously polarized, it is likely that over-polarization occurs due to a measurement lag.
Moreover, there arises the problem that it is necessary to provide an impedance analyzer, an AC signal source, an AC/DC separation circuit, a change-over circuit, and so forth in order to measure the piezoelectric constant during polarization, and the measuring device is complicated and is expensive.
To overcome the problems described above, preferred embodiments of the present invention provide a method of polarization-treating a piezoelectric body in which dispersion of the polarization degrees of respective piezoelectric bodies are greatly reduced, and a target polarization degree is attained very precisely.
Further, preferred embodiments of the present invention provide a method of polarization-treating a piezoelectric body in which over-polarization caused by a measurement lag is prevented.
According to a preferred embodiment of the present invention, a method of polarization-treating a piezoelectric body having the characteristic that current flowing through the piezoelectric body, caused by application of a direct current (DC) voltage to the piezoelectric body is increased, the method including the steps of applying a DC voltage to the piezoelectric body, measuring current flowing through the piezoelectric body caused by the application of the DC voltage, and stopping the application of the DC voltage at the time when the measured current value reaches a set level.
In the case where a piezoelectric body made of a material such as piezoelectric ceramics is polarized, as current flows through the piezoelectric body during the polarization, the current tends to be increased exponentially over time, as shown in FIG. 1, as an example.
One of the reasons for the current increase as shown in FIG. 1 is that with the progression of the polarization, the internal orientation of a crystal proceeds in the electric field direction, which causes the insulation resistance of the piezoelectric body to decrease over time, and the decrease of the insulation resistance causes the current value to increase. The current increase during the polarization appears more distinctly as the temperature becomes higher.
In the case of a piezoelectric body having current flowing through the piezoelectric body caused by the application of a DC voltage, is increased as shown in FIG. 1, the polarization degree of the piezoelectric body can be predicted based on the current value. In particular, since the current flowing through the piezoelectric body and the polarization degree have a high correlation, the polarization degree can be made to approach a target value by stopping the polarization at the time when the current value reaches a set level, and dispersion of the polarization degree can be reduced.
Moreover, it is unnecessary to measure the polarization degree (frequency characteristics) during polarization as conventionally required. It is only necessary to measure the current. Thus, the measurement time can be significantly shortened, and over-polarization, caused by measurement lag when many piezoelectric bodies are polarized, are prevented.
In addition, since it is unnecessary to measure the frequency during polarization, the apparatus for polarization according to preferred embodiments of the present invention is greatly simplified.
According to preferred embodiments of the present invention, either method of in-liquid polarization or in-air polarization may be used, since measurement of the frequency characteristics of a piezoelectric body is unnecessary.
FIG. 2 shows the variation of the polarization degree of a piezoelectric body in the steps ranging from polarization to aging to ordinary temperature restoration.
As seen in FIG. 2, the polarization degree increases to the maximum during the polarization, and is reduced by the aging, and is partially restored in the ordinary temperature restoration so as to be stabilized. The maximum polarization degree xcex94f1 in the polarization and the maximum current value (current limit value) iL at the maximum polarization degree xcex94f1 have a high correlation. Further, the maximum polarization degree xcex94f1 and the stable polarization degree xcex94f2 obtained after the ordinary temperature restoration have a high correlation. Accordingly, the current limit value iL and the stable polarization degree xcex94f2 obtained after the ordinary temperature restoration have a high correlation. In the above example, the polarization degree of a piezoelectric body is determined based on the frequency difference xcex94f between the resonance frequency fr and the anti-resonance frequency fa. A piezoelectric constant such as an electromechanical coupling coefficient K, a center frequency, or the like may be used to determine the polarization degree.
FIG. 3 shows a correlation between the current limit value iL in the polarization and the polarization degree xcex94f2 obtained after the application to the aging to the ordinary temperature restoration, following the current value reaching the current limit value. The correlation was determined under the following conditions:
piezoelectric body: PZT block (thickness 8 mm)
polarization voltage (inter-electrode voltage): 8.7 kV
polarization temperature: 200xc2x0 C.
aging temperature: 200xc2x0 C.
When the current values are expressed as the logarithmic values as shown in FIG. 3, it is seen that the current value (logarithmic value) and the polarization degree are substantially in proportion to each other in the range of the current limit value of about 2 mA to about 10 mA.
Preferably, the set level is determined based on a correlation between the polarization degree of the piezoelectric body obtained immediately before the application of the DC voltage is stopped and the stable polarization degree of the piezoelectric body obtained after the piezoelectric body is aged, and then, the temperature of the piezoelectric body is restored to an ordinary temperature, following the stop of the voltage application. Thus, the final polarization degree can be controlled at a target value very accurately.
For example, if a target final polarization degree xcex94f=3.0 kHz is desired, the polarization is stopped at the time when the current value becomes about 3.5 mA, and after aging, the temperature is restored to an ordinary temperature.
When the values of currents flowing through piezoelectric bodies are increased, voltage drop is generated in current limiting resistors which are provided for preventing over-current and are connected in series with the respective piezoelectric bodies, so that the voltages applied to the piezoelectric bodies are reduced, respectively. The reduction of the voltages causes the problems that the polarization rates of the piezoelectric bodies are reduced, and a desired polarization degree can not be obtained.
Accordingly, preferably, a voltage drop in a current limiting resistor is calculated based on a current value flowing through the piezoelectric body, and the voltage drop is added to an initial applied voltage.
That is, an applied voltage is determined according to the following calculation expression:
applied voltage=initial voltage+current valuexc3x97current limiting resistance
By keeping constant the voltages applied to the respective piezoelectric bodies at any time, dispersion of the polarization degrees of the piezoelectric bodies, caused by dispersion of the applied voltages, can be solved. In this method, the polarization condition (voltage) can be kept constant, in addition to the control of the polarization degree. Therefore, dispersion of the polarization degree is further reduced.
Other features, elements, characteristics and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.